This question is finally answered. I reposted this since the last thread contained unreltaed discussions, I figured this way it would be easier to see the issue and get right to the resolve without scrolling through the meat of the other thread. I have been going through this issue with Adobe and Wacom for months trying to figure it out,

In the end it is a combination of my system windows vista64, with two monitors, undocking the stage from the main flash interface, putting the stage on my Cintiq, and keeping the Flash interface window on my primary monitor that causes the brush sensitivity to disappear when the brush size is changed.

It is a bug only for PC, only (as far as I know) when the above steps are taken.

Unfortunately this is how I like to work since my Cintiq is a 15X I want to maximize my work space.

It's a bug and I hope they get around to fixing it.

In the meantime the workaround is to have the wacom preferences window open and when you change brush sizes, just draw in the test area of the Wacom preferences window and it will reset sensitivity until you change brush sizes again. It sucks to have to do everytime, but at least it works.

Or go get a bigger cintiq and keep the stage docked! I guess it's time to upgrade.